Liquid crystal display devices have been used for watches and electronic calculators, various measuring apparatuses, automotive panels, word processors, electronic notebooks, printers, computers, televisions, watches, advertising displays, etc. Typical examples of a liquid crystal display mode include a TN (twisted nematic) mode, a STN (super twisted nematic) mode, a vertical alignment mode using TFT (thin-film transistor), an IPS (in-plane switching) mode, and the like. Liquid crystal compositions used for these liquid crystal display devices are required to have stability to external factors such as moisture, air, heat, light, and the like, exhibit a liquid crystal phase (a nematic phase, a smectic phase, a blue, phase, and the like) within as wide a temperature range as possible including room temperature as a center, and have low viscosity and low drive voltage. Further, each of the liquid crystal compositions is composed of several types to several tens types of compounds selected for giving optimum values of dielectric anisotropy (Δ∈) and refractive index anisotropy (Δn) for a display device.
A horizontal alignment-mode display such as a TN mode, a STN mode, or an IPS mode uses a liquid crystal composition having positive Δ∈. There is also reported a driving method in which a liquid crystal composition having Δ∈ positive As is vertically aligned with no voltage applied, and a display is realized by applying a transverse, electric field, and the necessity for a liquid crystal composition having positive Δ∈ is further increased. On the other hand, an improvement in response speed is required for all driving methods, and a liquid composition having lower viscosity than present is required for solving this problem. In order to produce a liquid crystal composition having low viscosity, it is effective to decrease the viscosity of each of polar compounds constituting the liquid crystal composition. Also, when a liquid crystal composition is used for a display device or the like, it is required for the liquid crystal composition to exhibit a stable nematic phase within a wide temperature range. In order to maintain a nematic phase within a wide temperature range, each of the components constituting the liquid crystal composition is required to have high miscibility with other components and a high clearing pint (T→i).
In order to produce a compound having high T→i, it is known to be preferred to introduce, three or more ring structures such as a 1,4-cyclohexylene group or 1,4-phenylene group. On the other hand, in order to produce a compound having low viscosity, a compound having a plurality of ring structures directly connected to each other without through a connecting group, that is, a compound called a directly-connected ring system, is considered to be preferred. However, a directly-connected ring system compound having three or more ring structures and positive Δ∈ generally has high crystallinity and often has low miscibility in a liquid crystal composition. In order to resolve this problem, compounds having various connecting groups introduced therein have been investigated. It becomes clear that viscosity is slightly increased by introducing a connecting group, but miscibility in a liquid crystal composition can be improved (Patent Literatures 1 to 8). However, a molecule having e connecting group generally has high viscosity and the problem of significantly decreasing T→i. Patent Literature 9 gescribes a compound below as a compound having low viscosity and high miscibility in a liquid crystal composition. However, the compound does not have satisfactorily high T→i.
